1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display device, and more particularly, to an OLED display device which can improve brightness and color coordinate characteristics in all emission wavelength ranges using a simple structure and process and thus enhance light extraction efficiency and color reproducibility, by controlling thicknesses of first and second refraction layers disposed between an electrode and a transparent substrate.
2. Description of the Related Art
Light efficiency for light emitting devices, particularly flat panel display devices such as OLED display devices, is classified into internal efficiency and external efficiency. The internal efficiency depends on photoelectric conversion efficiency of an organic light emitting material. In addition, the external efficiency, called light extraction efficiency, depends on a refractive index of each layer constituting an organic light emitting diode. The organic light emitting diode exhibits relatively lower light extraction efficiency, i.e., the external efficiency than other display devices such as cathode-ray tubes or PDPs, and thus there is much room for improvement in characteristics of the display device such as brightness, life span, etc.
The biggest reason that the conventional organic light emitting diode has a lower light extraction efficiency than other display devices is because total reflection occurs at an interface between an ITO electrode layer having a high refraction index and a substrate having a low refraction layer when light is emitted through the organic layer at more than a critical angle, thus preventing extraction of the light. Therefore, due to the total reflection at the interface in the organic light emitting diode, only about a quarter of the light actually generated from an organic emission layer can be extracted outside.
An example of a conventional OLED display device for preventing a decrease in light extraction efficiency is disclosed in Japanese Patent Publication No. 63-314795. The OLED display device includes a substrate having a projecting lens. However, because a single pixel has a very small area, it is difficult to form the projecting lens for collecting light on the substrate.
To solve this problem of the OLED display device, an OLED display device having an optical microcavity is disclosed in Japanese Patent Application Laid-open Nos. 8-250786, 8-213174 and 10-177896. The OLED display device has a multi-layered semi-transparent mirror formed between a glass substrate and an ITO electrode, and the semi-transparent mirror serves as an optical resonator together with a metal cathode also serving as a reflective plate. Here, the semi-transparent mirror has a multi-layered structure by alternately stacking a TiO2 layer having a high refractive index and a SiO2 layer having a low refractive index, and an optical resonance is achieved by controlling reflections between the layers. However, such an optical resonator requires as many layers for the semi-transparent mirror as possible to improve refraction characteristics, and the number of layers and thickness of each of the layers have to be accurately optimized to control the reflection at a specific wavelength. For this reason, a process for fabricating the OLED display device may become complicated.